Personally, I like unslabbed coins better than slabbed. I do on occasion buy slabbed, especially for more expensive coins. The following three are generally accepted as the best, and I tend to agree.
#1 PCGS
#2 NGC
#3 ANACS
There's a whole plethora of new slabs, from grading companies that I would best describe as disreputable. At any rate, you will be better off by sticking with the three aforementioned services, especially when you decide to sell. Having said that, you still need to be selective. Eye appeal, whether slabbed or not is extremely important. 2 similar coins graded by the same service, with identical grades can be miles apart when it comes to eye appeal. If it looks nice to you, it will usually look nice to others.